In the past years, there has been a rapid growth in the power electronic industry. This industry involves the use of power electronic controlled switches. The function of these switches is to sequentially close and open a conductive link between two electrodes under the command of an electric signal. The actual technology uses semiconductor substrates to produce these switches, therefore the switching time thereof is much faster than switching time of the best mechanical relays. Because of that, the power semiconductor switch is attractive for electric power flow control. In these systems, semiconductor switches are normally grouped with passive elements in a defined configuration and connected between an electrical power source and a load or another electrical power source. Also, a series of voltage and current sensors are provided to supply information to a control unit where said information is analysed. Based upon the analysed information, switching commands are transmitted from the control unit to the switches so that large amount of electrical energy can be exchanged between sources in a controlled manner. These systems are better known as power converters.
One common application of the power converter is for driving an electrical motor. In this type of application, a DC voltage source is generally used as the power supply and a power converter is used to convert a DC voltage into polyphase AC voltage to supply and control a polyphase AC motor. These DC to AC power converters are also commonly referred as inverters.
In order to supply high power to an electrical AC motor, the power converter requires switches that can withstand high voltage and high current. In most known configuration, the power converter operates the switches in hard switching mode. In this mode, the switch goes from a blocking state to a full conducting state in two steps. In the first step, the current rises through the switch to reach the load current as there is still a blocking voltage across the switch. In the second step, the load current circulates through the switch while the blocking voltage falls until a full conduction voltage drop is reached across the switch. To go from full conducting state to a blocking state, the same steps are achieved in reverse.
During these two steps, the current-voltage product is very high and generates switching power losses within the electronic elements. These switching power losses are added to the conduction losses of the switches and they both generate heat. In order to avoid the destruction of the electronic elements, their temperature must be kept under a critical level. The cooling of the electronic components is then achieved by means of a heat exchanger joined side by side with the switching devices to release the thermal energy. The size of this heat exchanger depends directly on the amount of heat losses that is generated which itself vary proportionately with the switching frequency. On the one hand a converter operating at a low switching frequency requires a smaller heat exchanger. On the other hand, a higher switching frequency is desired to reduce the size of the filter components and the audible noise. Therefore, a compromise has to be made in the choice of the operating switching frequency. To reduce the size of the heat exchanger and the filter components, to reduce the noise, and to increase the efficiency of the power converter, the conduction and the switching losses have to be reduced.
The today most commonly used switching elements in power converters are gate capacitance transistors like Mosfets, IGBTs or MCTs because they are easy to operate. The conduction losses of these switching elements are in relation with their physical properties, with their size and with the fact that several of them can be mounted in parallel. Accordingly little or nothing can be done for reducing these conduction losses by acting on the way the switching elements are controlled. For example, it is not possible to act upon the conduction losses of Mosfets or IGBTs by working upon their control when their gate input capacitance is fully charged. But it is possible to reduce switching losses in cases where the switches are operated in such way that one can still act upon the switching losses by working on the way the switches are controlled. Thus, the switching losses can be further reduced by increasing the switching speed with the proper electrical adjustment such as an appropriate choice of a gate resistor for an IGBT or Mosfet. However, the power electronic switching devices are then submitted to high transient voltage spikes caused by abrupt changes in the current that flows through the stray interconnection inductance located within the power converter assembly. These voltage spikes are added to the bus voltage. A voltage combining the voltage spikes and the bus voltage are then applied across the switch during the turn-off phase. If the magnitude of the voltage spike becomes too high, the switching electronic elements within the power converter may be destroyed. In addition, the voltage spike generates noise that affects other components in the circuit through the stray capacitance elements and it also adds to the EMI (Electromagnetic Interference) problem.
Also, just before the turn-off phase, it should be noted that the energy stored in the magnetic field of the stray interconnection inductance will have to be released. This energy is eliminated through heat losses within the switches. In an article entitled “Losses due to stray Inductance in Switch Mode converters”, by M. Fasching and published in EPE Journal, volume 6, No. 2, pages 33-36 of September 1996, it is explained that additional power losses in a converter can be linked to the stray interconnection inductance when the switched-on current is different from the switched-off current in the same period, these losses contributing significantly to the total losses of the power converter.
In order to reduce the voltage spike and to reduce the switching losses of the converter, different techniques may be applied.
First, a clamping device may be connected across the switches to suppress the voltage spike during their turnoff phase. In doing so, the clamping device diverts and keeps the energy that was stored in the stray interconnection inductance just before the turn-off phase. It should be noted that when the clamping device is in operation, the switch does not control anymore the falling rate of the current. This falling rate depends on the voltage applied on the stray interconnection inductance and depends on the value of the stray interconnection inductance. The value of this voltage is representative of the amplitude of the voltage spike that can be withstood. Also, when the clamping device is in operation, energy is pumped out from the DC supply source into the clamping device. Thereby, the addition of a clamping device increases turn-off losses and the switch looses its control on the falling rate of the current. The more the voltage spike is suppressed, the higher the amount of energy to be disposed of by the clamping device is. If this energy is eliminated through heat losses, then it affects the efficiency of the power converter. According to recent developments in the state of the art, clamping devices containing additional components to return the trapped energy back into the DC voltage supply source are used. Then, the efficiency is not affected but the system is much more complex.
Secondly, instead of increasing the turn-off speed of the switches which causes voltage spikes, energy recovery snubbers may be used to absorb high switching losses and return this energy to the supply source. Then, the frequency at which the source voltage is switched can be increased. Thus, the efficiency of the converter is improved and voltage spikes are limited, but the current falling rate is still not controlled by the switch. In the article entitled “Toward 99% Efficiency for. Transistor Inverters” by I. Takahashi and al., published in IEEE Industry Applications Magazine, Volume 2, pages 39-46 of November/December 1996, there is shown the use of an energy recovery snubber circuit to improve the efficiency of the converter as mentioned above.
Both of the techniques described above improve the power converter but they add components to the circuit whereby increasing the complexity of the whole assembly.
If the use of clamping devices or snubbers is not desired in applications where voltage spikes cannot be suppressed, it is therefore necessary to use high voltage power electronic switching devices that can support the bus voltage plus the voltage spike during the turn-off phase. Unfortunately, this solution misuses the power switching capability of the device, requires bigger assembly to operate with the larger semiconductor chips and produces higher conduction losses because the size of the chips that are used and their conduction losses both increase with its voltage withstand capability. It is therefore strongly desired to reduce the stray interconnection inductance within the power converter assembly to reduce the magnitude of voltage spikes imposed on the switching devices, to increase the switching speed, to avoid the use of clamping devices or snubbers, to control the current falling rate by means of the switch, to reduce the losses associated with the stray interconnection inductance itself, and to reduce the size of the power converter.
The small stray interconnection inductance is obtained with good wiring structure within the converter. In large module, several semiconductor chips are mounted in parallel to increase the current switching capability. The wiring of these semiconductor chips has to be done in a specific manner to prevent oscillations. Bad wiring produces oscillations between the chips during turn-on and turn-off phases because of the stray interconnection inductance that links all of the chips. This problem must be considered in the design of a new module.
Known in the art, there are U.S. Pat. Nos. 5,616,955, 5,574,312, 5,563,447, 5,541,453, 5,523,620, 5,512,790, 5,471,089, 5,459,356, 5,457,604, 5,444,295, 5,424,579, 5,347,158, 5,170,337, 5,043,859, 4,907,068, UK patent No. 2 261 768 and European patents Nos. 621 635, 427 143, and articles entitled “A Novel Low-Profile Power Module Aimed at High-Frequency Applications”, published in the ISPSD Proceedings, 8th International Symposium on Power Semiconductor Devices and Ics, pages 321-324 of May 1996; “Latest technology Improvements of Mitsubishi Modules”, published in IEE Colloquium (Digest), #146, P.5/1-5/5 1996; “Reliable 1200 Amp 2500 V IGBT Modules for Traction Applications”, published in IEE Colloquium (Digest), #81, pages 3/1-3/13 1995; and “Advanced Power Module using GaAs Semiconductors, Metal Matrix Composite Packaging Material, and Low Inductance Design”, published in IEEE International Symposium on Power Semiconductor devices & IC's, pages 21-24, 1994.
In these documents, different embodiments are proposed to reduce the stray interconnection inductance within the internal part of a package that contains one or more semiconductor switching devices. However, these documents do not teach nor show how to reduce the stray interconnection inductance resulting from the wiring that connects each package to two DC voltage terminals decoupled by a capacitor.
Also known in the art are U.S. Pat. Nos. 5,430,326, 5,202,578, where modules with particular external connecting means arrangements are proposed for power semiconductor devices. With these modules, a power converter assembly is provided with bus bars and modules having interconnections length reduced to a minimum, thus reducing the stray interconnection inductance outside these modules. These patents do not teach nor show how the stray interconnection inductance within the module can be reduced.
Also known in the art is an article entitled “Bus Bars Improve Power Module Interconnections”, published in Power Conversion & Intelligent Motion: The Fusion of Power & Motion Technology & Applications, volume 21, number 4, pages 18-25, April 1995. In this article, a wiring technique is presented which employs laminated bus bars to interconnect power modules and the capacitor in converter assemblies. Using this technique, a low stray interconnection inductance is realized. However, this document do not teach nor show how the stray interconnection inductance within the module can be reduced.
Also known in the art are U.S. Pat. Nos. 5,528,073, 5,493,472, 5,414,616, 5,313,363, 5,142,439, 5,132,896 and Japanese patent No. 6225545. These patents disclose power converter assemblies each built with the manufacturer's power semiconductor switching modules, particular terminal links and a capacitor. These assemblies are made with short conductive interconnections having a particular arrangement so that the interconnection inductance outside of the modules, and including the capacitor, is low. An overall low interconnection inductance is achieved with these assemblies but these patents do not teach nor show how the stray interconnection inductance within the module can be reduced.
All of the above mentioned patents and documents give only a partial solution for reducing interconnection inductance.
Also known in the art is U.S. Pat. No. 4,670,833. In this patent, the complete topology of a power converter circuit is disclosed. The inventor discloses a new switching module provided with connecting means and arranged with a pair of DC terminals made of two layer conductive plates separated by an insulating layer and connected directly on the switching module thus reducing the stray interconnection inductance. An overall low stray interconnection inductance is achieved when a smoothing capacitor is connected directly on the two layer conductive plates. In this patent, the module containing the semiconductors switches, the capacitor and the DC terminals are not part of a unique package.
Also known in the art is U.S. Pat. No. 4,755,910, describing a packaging unit for encapsulating electronic circuitry. This invention is a circular electronic circuit board provided with a plurality of studs and at least one central opening. The studs are divided into two groups, a first group being located at the periphery of the circuit board, and a second group being located at the periphery of the central opening. Also, a multi-layer capacitor forms a cover superimposed on top of the circuit. This capacitor brings the supply voltage to the electronic circuit board via the two groups of studs. With this particular arrangement, the supply voltage travels along a distance that is at the most only the half of the distance travelled by the supply voltage in an electronic circuit board with the same area, its two supply voltage electrodes being located side by side at the periphery of the electronic circuit board. The wire inductance and the wire resistance are therefore reduced for each travelled path in the circuit lines. This invention is an improvement for electronic integrated circuit that uses multiple signal lines for logic transmission where a single wire inductance can cause noise that interferes with the logic level interpretation. However, this invention does not teach nor show how to provide a low interconnection inductance power converting module.